US2404677A - Fractional distillation - Google Patents

Description

July 23, 1194s,"

VFina oat, `29;

w. Vc. WILSON n marrow. Dx'snpwrml# Town l rz; c Trauer/NG Patented July 23, 1946 FRACTIONAL DISTILLATION Warren C. Wilson, Rahway, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application October 29, 1941, Serial No; 416,980

(Cl. 19d-94,)

9 Claims.

This invention relates to the fractionation of vapor mixtures and more particularly relates `to controlling the temperature .of the top of a fractionating tower by controlling the temperature of the reflux liquid introduced into the top of the fractionating tower.

In the fractionation of vapors containing different constituents such as hydrocarbon vapors `it is usual practice to control the temperature at the top of the fractionatng tower by regulating the quantity of reflux liquid introduced into the top of the fractionating tower. The reflux liquid may be cooled in various ways but if the rate of reiiuxing is held constant and the temperature is controlled by regulating the quantity of water passed to the coolers there is danger of vaporization of the water and by loss of water due to vaporizaton corrosion of the heat exchange coil or surface will result. Water used in these coolers is in many cases salt water which is more corrosive at h'igher temperatures.

According to my invention, the reflux liquid is withdrawn from the fractionating tower at a constant rate and passed in indirect heat exchange with fresh feed and another medium to recover heat and then the partially cooled reflux liquid is passed to another heat exchanger where cooling water isused. When using the reflux liquid as a heat exchange medium, it is desirable to obtain maximum heat recovery before 'cooling with water.

According to my invention, the danger` of vaporization of the cooling water and resultant corrosion of parts of the heat exchanger is avoided by holding substantially constant the quantity of water passing to the cooler or coolers and regu- 'lating the temperature of the reflux liquid by lay-passing a portion of the partly cooled reflux liquid around th'e heat exchangers or coolers.

According to my invention maximum heat is Y recovered from the reflux and put into the fresh feed without running the danger of operating the cooler at salt water temperatures considerably in excess of those required to prevent corrosion. In order to fractionate the vapors it is `necessary to vaporize the feed stock in the bottom of the tower and then remove heat somewhere up the tower. This heat removal can be accomplished by condensing vapors and refluxing the condensed liquid, but according to my invention, the heat is removed by removing a sidestream of hot liquid, cooling this liquid outside the tower, and then reinjecting it. The reinjected cooledliquid condenses a part of the vapors coming up the tower and in this way provides the necessary liquid reflux for fractionating the hydrocarbon vapors.

In my process the liquid sidestream is drawn olf and run through heat exchanger equipment and the recovered. heat is transferred to the fresh feed. In this manner the heat is recovered from the top of the tower. With my `invention itis desirable to circulate the maximum quantity of liquid from the tower in order to increase the heat transfer coelcient of the h'eat exchangers.

- Also, it is desirable to transfer as much heat as possible from the liquid to the fresh feed in order to secure maximum efliciency. In order to remove the correct amount of heat, however, it is necessary to provide a cooler or coolers in addition to the heat exchangerl equipment so that when the amount cf h'eat transferred `from the hot liquid to thev fresh feed is not sutlicient to give the required fractionation inthetower, the

additional heat may be removed by the cooler.

This means then that as the temperature` of the fresh feed is increased or as the heat exchangerequipment becomes fouled the load on the cooler is increased. i l i y If all of the liquid after passing `through the heat exchangers were to be passed to the cooler, it would then be necessary to regulate the quantity of Vwater going to the coolers. This could either be done by hand or by anautomatic control valve in the water line. However, if this were done there would be times when very little if any heat would have to be removed from the cooler and under` that condition the valve controlling the water to the cooler would be closed and without replenishing the supply of water the temperatureof the water in the cooler would gradually rise until it reached th'e vaporization temperature of water and it wouldv then be evaporated. With such a condition there would be severe corrosion of the cooler. In order to prevent this the equipment is laid out so that suicient water is always, flowing through the cooler to take care ofthe maximum heat load. Then if less than that maximum heat load is required the reflux liquid is by-passed around the cooler which means `that very little if any temperature rise would `occur onA the water going thrcugh the cooler and at no time would the temperature of the water reach the vaporization point and thereby start corrosion. Y

In the drawing: l Figure 1 represents one form of apparatus which may be used to carry out my invention; and

Figure 2 represents one form-of control valve whichmay loe-used.

character ID designates a line through which the J liquid to be fractionated is passed by pump I2. Where hydrocarbon liquids are to be fractionated,

4 o through line 84 is the lightest fraction separated from the liquid feed. Y

Returning now to the reflux liquid withdrawn from top trap-out tray 5ft, the reflux liquid is the oil feed comprises light naphtha or heavy 5 passed through line 64 by pump 85 and then naphtha or any other desired hydrocarbon fraction from lwhich it is desired to separate suitable fractions. VIi'iqu'id Y mixtures other lthan hydrocarbon mixtures may be fractionated according to my invention.

The liquid feed is passed through coil Illin heat exchanger I6 and then passed throughv :line I8 and through another coil 22 in a second heat exchanger 24 for preheating the vliq'uidifeed."P During passage through the C'ClS l4'ar`1d'-22, the liq- 15 'reflux Yliquid to the desired temperature.

uid feed is indirectly contacted with reflux liquid withdrawn from a fractionating tower@ .to be hereinafter described. If desired, onlyioneheat exchanger may be used or the feed may be passed through other heat exchangers for additional 2O Nheating. iI-hepreheatedliquidy *feed is 'passed through linetZIl andthrough the -heating coilv 2B in 'aheater 32/to'bring`the'liquid-feedto the dei -sired'temperatura through valved line 86 through the -heat eX- changers I6 and 24, previously described, where vthe reux liquid is cooled by indirect contact with lthe liquid feed introduced through line I0. If

. 10 ^desired, the refiux liquid may be in part or all v`by-passed around the heat exchangers IB and `1`2llthrough Icy-pass line 88 having a valve 9B.

The cooled reflux liquid is then passed through linegSZ; and! through cooler 94 for cooling the The coler'Ss is preferably a closed cooler. The tem- .fperature responsive device 63 in line $5, hereinbefore described, is connected by a line 95 to a control valve 95 in line S8 which communicates with line V92. Device 6-8 may control valve 96 by `-velectrical'rnean's, by air "pressure or,` in any suit- Aable mannerfor example, th temperature at il Geein line "56 is transmitted-to a temperatureconvtrol instrument such as a` Leeds and' Northrup i Inlet line "IU'is provided with a *valve-'Srand 25 fMicromaxjy which in turn controls'the pressure "outletlin25 is`provided with a valve .42 for byf passingrthe Lheat exchangers-` I t `and` 24Min which v"eveht'the -liquid'feed` is passed through kby-pass "jlineriic having a valve 42. The heated liquid on the diaphragm valve 96. By-regulating the 1 air pressure on the diaphragmfofvalve' in responseto temperature'changes,'the valve iseither *raised-or lowered and this in `turn :regulates 'the "feed-fromithe"heatingcoilZ'is introducedinto 30 flow: through lin S3. Other controfm'eansmay `"the'ilowerfportion oi" aA fractionating tower M "*through*line'V 46. ""The fractionating'tower 44 'providedwith trap-out trays'j'EZ and"54. The 'vheatedivapor'sfrom the liquid feed pass upwardvM13/through thefractionating tower and 'are Afracy 35 ".ti'onated` toL separate llighterA 'constituents from ""heavier"constituents, The Vfractionatingv tower `:mayheofianysuitable construction'such as a i"nubble'tower,

'j l"''lhe''trap-out trays are-'provided 'so "that dif- 40 "ferent 'fractions :may be `removedfrom :thefrac- L tionating'towerd The 'fraction having thehigh estlooiling range` is withdrawn from "thetrap-out Vtray 48. ..A lighter fraction is withdrawnffrom the-next higher" trap-out""tray "52 anda lighter' M"fraction is withdrawn from thetop traplo'uttrav T545 "The unva'porized heavier 'constituents lcoln *""lectthe'bottomiofthe fractionating tower i4 "iswitldrawnthrou'ghline 53 and may beA further "treated to .separate desired constituents theree ."Lfrom. "Ihe'" fraction 'withdrawn 'from trap-out tray '52 is 'passedthrough ,line'iiZ and maybe furl Alth'ertreated 4Las 'desired Vto separate" desired con ture 'of the reflux liquid introduced into the fracvtiona'ting'tower'li'as'willbe hereinafter described r'irrgreater detail. The overhe'ad'vapors arep'assed v'through' heat exchangers 'I2 vand "I4" where' Vthey nfares" indirectly contacted' Awith cooling water to be fused. VVThese control valves andL associated parts'are` well known equipment available on the v7market and furtherV detailed description ofthe equipment is not believed necessary. V

' Asshown in the drawing,"the temperature responsive device 63is a bulb or the like containing aliduid or liquids having desired vapor pressures at'-"the"cperating temperatures. `An increase in temperaturein line Alili results in ariincrease in line 95 to the pressure responsive valve-SBSVThe f valve- 96Y and 'associated parts shownir'Figure 2 will now v'be described.

The control` valve 9G comprises a valve housing 451- saiavaivefmc and a vaiveseat marvalveioo thas a '-valvestem'iIM "passingi'through agland 'packing H36. f The upper 'end ofstem I F34 :is secured to am-ovable f member ror'diaphragm I B8 ""at I I2. *Positioned between'idiaph'ragm I-8`and packing I'liik is a spring:surrounding'stem "IIL The diaphragm housing II 5 has a fixed-rear wall l il 81 and sidefwall I 22forrningA a cupshaped :cham-ber to which the-diaphragml is secured. Housing IIS is lrigidly secured, to the Valve hous- 55ing199 bymembersv I24andY-l26. Tube or line'S f .communicates withthe housing I I6.

From the above it will be seen thatias the: tem- .peratureat iincreasesithe liquid intemperature responsive device Will have .its temperaturefin- @F0` creasedrandl the pressure incline 95` williincrease.

." The. increaseinepressure will movediaphragm |08 down against the action ofspring IIIIA to close Y Y or paitlyfclose valve |00 and this will causel'the liquid to be passed through lineISU and cooling 5 couilaz inthe cooler s4 .to redutne temperature of thereux liquid. t

The cooled liquid from the coil I32`is` then passed through line 34 where it joinsthe'reflux 'liquid by-passed around the'coolerf'fSlI andjpasscondense and cool the vanors.1"The condensed 70"ing"through' line 93. 'The' reuxi'quids" then and cooled vapors are passedthroughline "I6 to a gas'"separatorl "18Minu 'which thefgases'jare separatedf fromf'"liuuidlzthe gases passing kf'overhead through linel "82 andr` theliduidvv being with-drawn through bottom line `IM.

passedA through line l|36 and introduced into the top portion "of" the fractionatingLtowefllliabove vvthe-top trapout 'tray' `-54, Preferably, thef'relatively `cooled-reflux' 'liquid 'ist' introduced' into a pressure' indevice S8 which is transmitted through ferential pressure valve |38, hereinafter described in greater detail, to insure satisfactory operation of the control valve 96.

If the temperature at 68 decreases the pressure in housing ||6 on control valve 96 will decrease and spring ||4 will force the diaphragm |08 upwardly to open valve |00. f

The cooling water for the cooler 94 is introduced through line |42A and overflows through line |44. According to my invention, the amount of water passing to the cooler 94 is not regulated or controlled but the cooler is kept full of water all of the time by using an excess of the maximum requirement. In this way corrosion of the apparatus due to vaporization of the water is eliminated. The temperature of the reflux liquid is controlled by ley-passing the cooler 94. In order'to obtain the desired temperature, all or a portion of the reflux liquid is by-passed around the cooler 94.

The cooling water used in the cooler 94 is preferably rst used for cooling overhead vapors from the fractionating tower 44, The cooling water may comprise any available water such as salt water, well water, etc. The cooling water is passed through line 46 by pump |48 and through the heat exchangers 14 and 12 where the cooling water indirectly contacts the vapors passing through line 66. During this heat exchange, the cooling water has its temperature raised a small amount. The cooling water is then passed through line |52 and introduced into the cooler 94 through line |42 as above described.

If desired, the Ifractionating tower 44 at its lower portion may be provided with a reboiler.

Diiferential pressure valve |38 will now be described. Two pressure conneotions |60 and |62 are taken as indicated, one on each side of cooler 94, 'Ihese pressure connections are connected by lines |64 and |66 to a differential pressure regulator instrument, such as is manufactured by the American Meter Company or the Foxboro Meter Company, and which is shown diagrammatically as including pressure chambers |68 and |10 connected by a line |12, Line |12 communicates with a valve memberI |14 into which air is sup plied by line |16. The air under pressure supplied through valve member |14 depends on the differential pressures in chambers |68 and |10 and is passed through line |18 to control valve |38. This control instrument regulates the air pressure on` diaphragm valve |38 either raising or lowering the valve in order to maintain a constant differential pressure across the cooler 94. This differential pressure regulator is preferably used in order to take care of the wide range in pressure drop which may occur acrosscooling coil |32 when only a very small part of the total liquid being circulated is run through the cooling coil |32 and when the major portion of the liquid is being run through the cooler. In order to operate valve 96 properly there must be a fairly constant differential pressure across the cooling coil. When a large portion of the stock is going through the cooling coil a pressure drop is builtl up by the resistance to the flow of this liquid. However, when only a small portion is going through the cooling coil there would be practically no resistance and therefore no pressure drop. Valve |38 merely holds this differential pressure constant. Other means may be used to accomplish this result.

While the apparatus may be used generally for the fractionation of liquid mixtures, it is especially adapted for the separation of hydrocarbon mixtures into desired fractions. One

example of fractionating hydrocarbon oils will beV given. A hydrocarbon mixture such as a heavy naphtha having an A. P. I. gravity of about 50 and having an initial boiling point of about 200 F. and a final boiling point of 435 F. is passed through the heater 28 and heated to a temperature of about 625 F. while maintained under pressure of about 50 lbs. per square inch gauge. The vapors are fractionated in the fractionating tower 44. Unvaporized residual oil withdrawn from the bottom ofthe fractionating tower 44 through line 56 has a boiling range of about 560 F. to about '100 F. and has an A. P. I. gravity of about 26.

The hydrocarbon fraction withdrawn from the lowest trap-out tray 48 has an A. P. I. gravity of about 36 and an initial boiling point of about 400 F. and a final boiling point of about 620 F. This fraction is withdrawn at a temperature of about 360 F. The fraction withdrawn from the trap-out tray 52 has an A. P. I. gravity of about 44.2, an initial boiling point of about 275 F. and a final boiling point of about 390 F. The lighter fraction withdrawn from the trap-out tray 52 is at a temperature of about 310 F. The reflux liquid withdrawn from the `topitrap-out tray 54 has an A. P. I. gravity of about 60, an initial boilingpoint of about F. and a final boiling point of about 320 F. The reux liquid withdrawn through line `64 is at a temperature of about 225 F. l

The vapors leaving the top of the fractionating tower are at a temperature of about 225 F. and after being condensed by passing through the coolers and'condensers 12 and 14, the distillate collected in the separator 13 forms the lightest fraction of the naphtha, This light distillate has an A. P. I. gravity of 62, an initial boiling point of about 60 F. and a final boiling point of about 290 F. In order to maintain the tempera* ture of the top of the tower at about 225 F. and further to obtain the desired amount of fractionation, the reflux liquid is returned to the top of the tower and the splash pan |31 at a temperature of about F. The ratioof the reflux to the vapors passing overhead is about 4 to l b-y weight. l

The cooling water is introduced into the cooler 64 at a substantially constant rate to maintain the cooler full of cooling water.

The reflux liquid lis'passed through heat ex- Ichangers i6 and 24 to recover heat from the reflux liquid.V The reflux liquid has its temperature reduced to about 200 F. after having passed through the heat exchangers I6 and 24. In this way the maximum amount of heat is recovered from the reflux liquid. 'Ihe temperature of the -reflux liquid is controlled by the temperature responsive device 68 in the outlet line 66 from the fractionating tower 44. If the vapors passing overhead through line 66 from the fractionating tower 44 are at too low a temperature, the reflux liquid is too cold and the temperature responsive device 68 moves the control valve 96 toward open position to permit by-passing of the reuxliquid through lines 98 and |36 around the cooler 94.

If the vapors passing overhead through line 66 are at too high a temperature, the reflux liquid temperature is too high in which event the temperature responsive device 68 effects movement of the control valve 90 in line 98 toward closed position and more of the reflux liquid is passed through line |30 and coil |32 in cooler 94,

7 The side streams taken through lines 58 and 52 may be passed to separate stripping towers to separate lighter constituents therefrom which are .preferablyA returned to the fractionating tower 44.

If desired, instead of controlling the process with a temperature responsive device at the top ofthe tower 44, a similar arrangement may be used to regulate the temperature at any place in the tower where the stock is to be circulated through a water cooler. e

Other hydrocarbon mixtures may be fractionated as for example hydrocarbon mixtures resulting from the cracking o-f hydrocarbon oils or heavier oils may be fractionated to separate them into desired fractions and my invention may be used in fractionating lthese mixtures. Or,V other mixtures may be fractionated.

While one form of apparatus has been shown for carrying out my invention and one Specific example has been disclosed, it is to be understood that these are by way of illustration only and various changes and modifications may be made without departing from the spirit of the invention.

I claim:

1. In the fractionation of mixed vapors in a fractionating Zone to separate desired constituents, the steps of removing reflux liquid from the upper portion of said fractionating zone at a constant rate, passing the reflux liquid through a cooler, passing a cooling medium through said cooler at a substantially constant rate in excess of the maximum cooling required and regulating the temperature of the reflux liquid to be returned at a constant rate to saidy fractionating tower by by-passing at least a portion of the reflux liquid around said cooler, the relative flow of the reflux liquid through and around said cooler being controlled by the temperature in the upper portion of said fractionating zone.

2,'In the fractionation of mixed vapors in a fractionating zone to separate desired constituents, the steps of removing reflux liquid from the upper portion of said fractionating zone at a constant rate, passing the reflux liquid in heat exchange relation with the liquid .feed passing to said iractionating zone to obtain maximum heat recovery from the reflux liquid, then passing the partly cooled reflux liquid through a water cooler, passing water through said cooler at a substantially constant rate in excess of the maximum cooling required and regulating the temperature of the reflux liquid to be returned at a constant rate to said fractionating tower by by-passing at least a portion of the reflux liquid around said cooler, the relative flow of the reflux liquid through and around said Water cooler being con- 8 trolled Vbythe ,temperature in the `upper -portion of said fractionating Zone,

3. A method of fractionating liquids comprising mixtures of different constituents which `comprises heating a liquid feed andintroducing it into a lfractionating zone, fractionating the vapors to separate lower boiling constituents from higher boiling constituents, passing vapors of lower boiling constituents overhead, withdrawing a relatively light liquid from the upper portion of said vfractionating zone atV a constant rate, passing the withdrawn relatively light liquid through a cooler, supplying a cooling medium to said cooler at a substantially constant rate in excess of the maximum cooling required, by passing at least a portion of the relatively light liquid around said cooler, returning the combined cooled relatively light liquid at a constant rate to said fractionating zone as reflux liquid, and controlling the relative amounts of the relatively light liquid passing through and around said cooler bythe temperature in the upper portion of said fractionating Zone.

4. A method according toclaim 1 whereinthe reflux liquid withdrawn from' the upper portion of said fractionation zonelis passed in indirect heat exchange with fresh liquid feed of said fractionation zone, before it ispassed through said cooler. 'n

5. A method according to claim 1 whereinv the differential pressure across said cooler vis kept substantially constant independent of the amount of reflux liquid passed through said cooler.

6. A method according to claim 3 wherein the relatively light liquid withdrawn from the upper portion of said fractionating zone is ypassed* in indirect heat exchange with the liquid feedbefore itis passed through said cooler to obtain maximum heat recovery from the relatively light liquid. y l

7. A method according to claim 3 wherein the relatively light liquid withdrawn from the'upper portion of said fractionating Zone is passed in indirect heat exchange with the liquid feed before itis passed through said cooler and wherein further thecooling medium is water and the'water before being passed to said cooler is passed in indirect heat exchange with the overhead vapors from said fractionating zone to cool and. condense the overhead vapors.

8.- A method according to claim 1 wherein hydrocarbon mixtures are fractionated to separate lower boiling hydrocarbons from condensate oil.

9. A method according to claim 2 wherein hydrocarbon mixtures are fractionated to separate lower boiling .hydrocarbons from condensate oil.

WARREN C. wILsoN.

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